1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same, and more particularly, to a read only or programmable non-volatile semiconductor device capable of storing charges in a gate insulation film and a method of fabricating the same.
2. Description of the Related Art
The semiconductor memories are categorized into a volatile type in which information is lost by power off and a non-volatile type in which information is retained even by power off. A typical example of the latter non-volatile memory is a flash memory capable of erasing all data in an area at once, so that the rewriting time can be reduced.
Recently, there has been proposed a multi-bit cell structure, which may have has a MNOS (Metal (gate)-Nitride-Oxide-Silicon) structure or a SONOS (Silicon (gate)-Oxide-Nitride-Oxide-Silicon) structure. These structures employ an ON (Oxide-Nitride) structure or an ONO (Oxide-Nitride-Oxide) structure as a gate insulation film just under the gate electrode. The multi-bit cell structure is implemented by locally storing a charge in a region in a nitride (Si3N4) film provided in the vicinity of the source and drain of a transistor and locally storing another charge in another region. The multi-bit structure increases the cell capacity and reduces the bit cost. For instance, two local regions for storing charges in the Si3N4 film enable a single cell to store two bits (2 bits/cell).
The floating gate structure conventionally employed in the flash memory has an electrically conductive polysilicon film, which is sandwiched between silicon oxide films and is used to store the charge. The floating gate structure has a disadvantage in that the charge may leak because of the use of the conductive polysilicon for storing the charge, and data may be lost. In contrast, the MNOS or SONOS structure does not have the above disadvantage because the charges are stored in the nitride compound that is electrically insulative.
FIGS. 1A and 1B are respectively cross-sectional views of a memory used to explain the method of fabricating a multi-bit memory cell having a gate of the SONOS structure, and a source and a drain having a buried bit line structure. More particularly, FIG. 1 shows a core area of the memory, and FIG. 1B shows a memory peripheral area.
In the core area, n-type diffused regions 102 used as bit lines and formed by diffusion of As are arranged at given intervals on the main surface of a p-type semiconductor substrate 100. Each section between the adjacent n-type diffused regions 102 is a channel region. A tunnel oxide film 103 having a uniform thickness is provided on the channel regions and the n-type diffused regions. A nitride film 104, an upper oxide film 105 and a not-shown control gate are laminated on the tunnel oxide film 103 in turn, so that the gate is formed by the four laminated layers. The nitride film 104 is electrically insulative, and electrons injected through the tunnel oxide film 103 are stored therein.
The conventional memory with the conductive floating gate is capable of storing one bit per cell because the electrons injected through the tunnel oxide film are spatially distributed in the floating gate so as to have a uniform electric field distribution and the whole floating gate serves as a charge storage region. In contrast, the memory with the gate of the MNOS or SONOS structure allows the electrons injected in the nitride film 104 of the insulator to be locally arranged and stored therein without being distributed, so that a multi-bit structure (many-valued structure) can be achieved. An ion implantation 101 is implemented in the p-type semiconductor substrate 100 in the core area for the purpose of adjusting the threshold value of each bit.
In the memory peripheral area (peripheral circuit area), well regions 106 are arranged at given intervals on the main surface of the semiconductor substrate 100. LOCOS regions 107 for device isolation are provided between the well regions 106, wherein the LOCOS regions 107 are formed by locally making a thin oxide film 108 thicker. The nitride film 104 and the upper oxide film 105 are laminated in turn on the oxide film 108 and the LOCOS regions 107.
The multi-bit cell thus structured may be fabricated by the following process. First, a first oxide film is formed on the semiconductor substrate 100, and the well regions 106 are formed in the peripheral circuit area by ion implantation. More specifically, the thin oxide film on the semiconductor substrate 100 is coated with a resist, which is then patterned. Thereafter, ions are implanted in the semiconductor substrate 100 with the patterned resist being used as a mask, so that the well regions 106 can be defined.
Next, the LOCOS regions 107 for device isolation are formed in the peripheral circuit area. This may be done by forming a SiN film on the semiconductor substrate and providing a resist thereon, which is then patterned. The SiN film is etched with the patterned resist serving as a mask for forming the LOCOS. Thereafter, the semiconductor substrate 100 is locally oxidized through openings, so that the LOCOS regions 107 are formed. After removal of the resist, the SiN film remaining in the core area is removed.
Then, a resist is provided and patterned, and ion implantation is carried out with the patterned resist. Ions are implanted through the openings of the patterned resist, so that the channel implantation region 101 for the adjustment of the threshold value can be formed in the semiconductor substrate 100.
Subsequent to the above, the resist and the thin oxide film are removed, and the tunnel oxide film 103 and the nitride film 104 for the charge storage are formed. A patterned resist is provided on the nitride film 104 and ions are implanted in the semiconductor substrate 101 through the openings of the patterned resist, so that the n-type oxide regions 102 are formed.
Finally, the resist is removed and the upper oxide film 105 is deposited on the nitride film 104.
The multi-bit cell of the SONOS structure can be independently programmed for each of the bits of the cell. Thus, the cell capacity can be increased and the bit cost can be reduced.
However, the conventional multi-bit cell of the MNOS structure or SONOS structure has the tunnel oxide film having the uniform thickness, which determines the storage location of the charge due to the electron-injected position depending on the potential inclination between the source and drain regions. As the amount of charge stored in each bit increases in the progress of programming, the bit region that stores a large amount of charge tends to gradually spread towards the channel center from the vicinity of the drain in the nitride film. The spreading of the bit region causes another bit that stores only a small amount of charge to equivalently function to storage a large amount of charge that exceeds the real amount of charge and to raise the threshold value of that bit. This may cause a data read error.